1. Field of the Invention
The present invention relates to a wavelength division multiplexing transmission system, and more particularly to a wavelength division multiplexing system for multiplexing and transmitting wavelength division multiplexing signals where signal lights with different signal bandwidths are wavelength-division multiplexed, or receiving and demultiplexing wavelength division multiplexing signals.
The present invention also relates to an optical transmitter for multiplexing a plurality of signal lights and transmitting the same as wavelength division multiplexing signals and an optical receiver for receiving the wavelength division multiplexing signals and demultiplexing the same into signal lights with a respective wavelength, which are included in the wavelength division multiplexing transmission system.
2. Description of the Related Art
Recently the capacity of Wavelength Division Multiplexing (WDM) transmission systems is increasing. A method of increasing capacity is a method of increasing the number of wavelengths to be multiplexed and increasing the transmission speed (bit rate) of the signals with respective wavelengths. At the moment, 10 Gbit/s WDM transmission systems have already been commercialized, and research and development of 40 Gbit/s optical transmission systems are in-progress.
However, to introduce a 40 Gbit/s WDM transmission system, a partial upgrade of switching a part of a conventional 10 Gbit/s WDM transmission system to a 40 Gbit/s is considered in terms of the installation cost and upgrading in-services, rather than installing a WDM transmission system of which signal lights with all wavelengths are 40 Gbit/s. In other words, upgrading to a WDM transmission system where a 10 Gbit/s and 40 Gbit/s systems coexist is being considered.
As the number of wavelengths to be multiplexed increases, the wavelength interval between signals becomes closer, and at the moment, a system with a 50 GHz wavelength interval (frequency interval) is commercialized for 10 Gbit/s systems, and the use of a 100 GHz wavelength interval (frequency interval) is under consideration for 40 Gbit/s systems.
For this increase in the density of wavelengths, a method of multiplexing/demultiplexing lights by an arrayed wave guide grating (AWG) filter and a multi-layer film filter at a 1:N channel optical multiplexing/demultiplexing module and then further multiplexing/demultiplexing this light using an interleaver is used. For the index of increasing the density of wavelengths, spectral efficiency to indicate the bit rate per unit frequency is used. In the case of a 10 Gbit/s system, this bit rate is 0.2 bit/s/Hz (=10 Gbit/s ÷50 GHZ), and in the case of a 40 Gbit/s system, this bit rate is 0.4 bit/s/Hz (=40 Gbit/s ÷100 GHZ).
An interleaver is an optical multiplexer/demultiplexer having a function to demultiplex a signal light group with a certain wavelength interval into even channels and odd channels, and generate a signal group with a double wavelength interval, or a function to multiplexing the even channels and the odd channels and generate a signal group with a ½ wavelength interval.
On the other hand, there is at WDM optical communication system to efficiently accommodate signal lights with a plurality of bit rates by arraying the signal lights with different wavelength intervals (see Japanese Patent Application Laid-Open No. 2002-112294, for example).
According to this WDM communication system, in the case of 40 Gbit/s signals, for example, four wavelength interval signal components are bundled into one channel, and in the case of 10 Gbit/s signals, two wavelength interval signal components are bundled into one channel, so that signals for each bit rate channel are generated. And 40 Gbit/s signals are arrayed at 100 GHz intervals, and 10 Gbit/s signals are arrayed at 50 GHz intervals, and they are transmitted respectively.
Before upgrading this system, in other words, in the system which wavelength-multiplexes and transmits 10 Gbit/s signal lights at 50 GHz intervals, if a part of the signal lights are directly changed into 40 Gbit/s signals, and these wavelength division multiplexing signals (WDM signals) are multiplexed/demultiplexed by a normal interleaver with 50 GHz/100 GHz intervals, the transmission quality deteriorates. This is because the spectrum width (bandwidth) of 40 Gbit/s signal lights is wider than that of 10 Gbit/s signal lights, so the signal components of 40 Gbit/s leak into the adjacent channels (cross-talk), and also because the spectrum of the 40 Gbit/s signals itself is restricted by the interleaver.
If an interleaver with a 100 GHz/200 GHz interval, which is used for 40 Gbit/s transmission, is used, cross-talk or transmission quality problems do not occur, but the spectral efficiency becomes lower, 0.25 bit/s/Hz, since 10 Gbit/s signal lights are also transmitted with 100 GHz intervals, which cancels out the effect of upgrading.
Also the signal wavelengths of a conventional system are arrayed in a grid with equal intervals as specified in ITU-T recommendations, so it is preferable not to change the conventional wavelength array, such as a 50 GHz interval or a 100 GHz interval, when upgrading.
Also the spectrum width of optical signals with respective wavelengths differ in the same way when the modulation scheme is different, such as RZ, NRZ and CSRZ, even if the bit rate is the same. Therefore this case can be considered in the same way as the case of increasing the speed of the bit rate.